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Airbus planes can't stall. Or can they?

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  • #16
    I mean, if normal law was active, it would keep commanding nose-down to lower a (maybe wrong) persistent too-high-AoA data, and even the pilots trying to apply full nose-up inputs would be futile becuase the stall protection overrides the pilots' inputs...

    Until the pilots hit the "gimme my frigging plane back" red button... except that there isn't one.

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
    --- Defend what you say with arguments, not by imposing your credentials ---

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    • #17
      Originally posted by Gabriel View Post
      I guess you can't have normal law with an unreliable AoA indication.

      In other words, more or less the same that happens with the unreliable airspeed, only that in this case the affected units are the ADRs.
      I think the system would rule the AoA value invalid under certain extreme circumstances, such as it did with AF447. Perhaps this is why the FAC's give up.

      The puzzling thing is that they are apparently thrown offline by sustained downward elevator yet they have nothing to do with pitch control and should require AoA only as a function of roll.

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      • #18
        Ahh, ok, I dug into the FCOM and found the FAC section…

        The FAC's have a fourth channel that governs the flight envelope protections, so they require AoA and are responsible for calculating maneuvering speed and the values for alpha floor and low energy warnings.

        Originally posted by Evan
        Overhead FLT CTL panels > FAC 1 to OFF, FAC 2 to OFF = you have the friggin plane (my conclusion based on the FCOM reversion law charts)
        Also wrong about this: as long as one of the FAC's remain valid, selecting them off will not defeat protections.

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        • #19
          Originally posted by Evan View Post
          Ahh, ok, I dug into the FCOM and found the FAC section…

          The FAC's have a fourth channel that governs the flight envelope protections, so they require AoA and are responsible for calculating maneuvering speed and the values for alpha floor and low energy warnings.

          Also wrong about this: as long as one of the FAC's remain valid, selecting them off will not defeat protections.
          In plain Spanish please? Or make it plain English?

          --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
          --- Defend what you say with arguments, not by imposing your credentials ---

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          • #20
            Originally posted by Gabriel View Post
            In plain Spanish please? Or make it plain English?
            Huh?

            The FAC's are the brains behind the speed protections. They trigger the SPEED warning and after that the Alpha Floor function. They also calculate Vsw and Vs speeds from current AoA, airspeed, altitude, thrust and CG. They also detect windshear.

            Why a sustained downward elevator would send them offline is puzzling. I think what happens is they are ruled invalid, probably, as you suggest, due to an unexpected set of performance parameters. But I'd love to know this for a fact.

            Once the FAC's are lost, there is no speed protection available and, by the book, you are in alternate law with no AoA protections either. It might also explain why there was no WINDSHEAR warning if one applies here.

            And of course, why these pilots didn't follow the standard GA procedure at the low energy warning is beyond me.

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            • #21
              Originally posted by 3WE View Post
              So....

              Stall training is finally adjusted in a manner that makes you happy...

              ...and we have what might be a stall-related crash in Russia, the Gabriel post on a 737 near-stall story at another forum and the Gabrield Airbus actual stall story here...

              ...I dunno- maybe it was better before, even though Pinnacle, Colgan and Air France were eye-rollers...
              Actually, we have two incidents there stemming resulting from the same pilot error: failure to use the TO/GA button or detent where it is called for.

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              • #22
                Sorry. I meant about the last line:

                Also wrong about this: as long as one of the FAC's remain valid, selecting them off will not defeat protections.
                Does it mean that a FAC can remain valid even after turning it off, and hence even turning al of them off would not by itself "give you the friggin plane"?

                So what does it take? An axe?
                Let me guess: Turning 2 ADRs off.

                --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                --- Defend what you say with arguments, not by imposing your credentials ---

                Comment


                • #23
                  Originally posted by Gabriel View Post
                  Sorry. I meant about the last line:


                  Does it means that a FAC car remain valid even after turning it off, and hence even turning al of them off would not by itself "give you the friggin plane"?
                  Apparently yes. As long as one is valid, even if both are selected off, the protections remain.
                  I'm guessing this is because the FAC's have four independent channels and the one governing speeds and flight envelope functions remains active when the FAC is selected off.
                  Originally posted by Gabriel View Post
                  So what does it take? An axe?
                  Let me guess: Turning 2 ADRs off.
                  Again, I only can say this in theory, but the law reversion flowcharts indicate that taking two ADR's off (pushbuttons AND rotary switches to off) would do it. Or taking both ELAC's or all three PRIM's offline.

                  EDIT: all three PRIM's or all three ADIRU's offline should get you DIRECT law via the SEC's.

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